Swim & Blaze have expressed this/summed it up perfectly.
Cunnilingus is considered a low-risk behavior compared with vaginal or anal intercourse, but it's not completely risk-free. Since you are no where near the cervix where these infections live, you're very unlikely to get GC (gonorrhea) or chlamydia. The person performing DATY can get vaginal lining disease conceivably like Herpes, HPV (Human Paipilloma Virus) with Type 16 and others correlating to causing cervical cancer in women, and syphilis which is pretty rare in 2012 but not prior to 1945 when Pen hit the market.
If you are the one with the "Y" and being given head, then you have a relatively small risk that you could get HSV-1 (herpes) from the person giving you head if he/or she had herpes lesions on the lip. Not all blisters on the lip are herpes by any means although I've seen plenty of people who worry they are.
The only way to decrease the relatively low risk from DATY is to use things that are cumbersome, and most people find kills the pleasant sensation from it. Condoms and dental dams. For people allergic to latex, dental dams are made in silicone. Saran wrap can work just as well. The problem with these is that they are not likely to stay on at all in reality, and it's a cold day in July when anyone wants to use them. A dental dam would reduce the chance of getting Herpes or HPV if the person performing DATY had it.
I wouldn't be very afraid of DATY of BBBJ at all. The enjoyment so heavily outweighs the infinitely small risk unless either partner knows of a significant lesion on one of them.